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An Upside to China’s Air Pollution: More Snowfall in the Sierras

 
Researchers uncover a link between dust storms in Asia and weather in California

Sierra Nevada Mountains and Los Angeles AqueductA storm of yellow dust darkens the skies above Beijing, an increasingly familiar phenomenon blamed on the disappearance of Asian forests. A week later, in California’s Sierra Nevada mountain range, where annual precipitation levels are expected to decline as the climate changes, a snowstorm delights skiers.

The storms are starkly different and separated by thousands of miles, but scientists have discovered that they are linked.

Researchers from the Scripps Institution of Oceanography and the University of California, San Diego, have discovered that dust storms in Asia could help douse the Sierras with snow, bolstering California’s economy and rejuvenating its environment.

Spring and summer snowmelts in the Sierras provide fresh water for 25 million people, for wildlife and millions of acres of farmland, as well as for hydropower that meets up to 15 percent of the state’s electricity needs.

In experiments that began in 2009, the scientists discovered that dust that glides on high-altitude jet streams from Asia over the Pacific Ocean plays a substantial role in seeding snowflakes that fall over the Sierras.

Industrial pollution, bacteria, heavy metals, dust and other aerosols flow freely from Asia to California. Research suggests that as much as one-third of the airborne lead in the San Francisco Bay Area wafted over from Asia.

Raindrops and snowflakes generally cannot fall out of a cloud unless there is a floating seed husk, piece of pollen, speck of dust or other aerosol that they can cling to and grow around.

“In order for water to condense out of the water vapor and into a droplet, it has to have a surface to condense on,” said Doug Collins, a chemist at the University of California who is involved with the Sierra studies. “Aerosols provide that surface.”

But different particles have different effects on clouds. Some trigger snow or rain, while others dissolve budding droplets, turning them back into mist and preventing or limiting precipitation.

In early 2009, the researchers studied two similar storms that occurred within a week of each other over the Sierras. The first was relatively dust-free. But the second fell from clouds filled with dust from a storm that had torn through China one week earlier, and it dumped 40 percent more snow than the earlier storm.

To test whether the heavier snowfall was related to the dust, the researchers took to the skies last winter in a twin-engine airplane equipped with sensors that counted dust particles and ice crystals. Ice crystals grow into snowflakes as they fall from the sky.

They found a close relationship between the levels of dust in the atmosphere and the number of ice crystals. “They’re in lockstep,” Collins said.

The research is part of CalWater, a project that started in 2009 and is intended to improve weather forecasting and modeling.

“There’s a lot of uncertainty in precipitation forecasting,” said Marty Ralph, a research meteorologist with the National Oceanic and Atmospheric Administration, one of the agencies that sponsored the research. “It’s quite a complicated process.”

This article also appears in the Bay Area edition of The New York Times.

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